The present invention relates to a flow sensor used for measuring the flow velocity or flow rate of a fluid flowing in a channel and, more particularly, a thermal flow sensor.
In a thermal flow sensor for measuring the flow velocity or flow rate of a fluid, a sensor chip having a flow velocity detection mechanism is arranged in a pipe to be parallel to the flow of a fluid to be measured. In the flow velocity detection mechanism, the spatial temperature distribution of a fluid is localized by the flow of heat emitted from a heat-generating body (heater). This localization is detected by a temperature sensor (indirect heated type), or a change in power or resistance occurring when the heat of the heat-generating body is deprived of by the fluid is detected (self-heating type), thus measuring the flow velocity or flow rate (examples: Japanese Patent Laid-Open No. 4-295724, No. 2-259527, No. 8-146026, and the like).
FIGS. 17A and 17B show a conventional flow sensor. This flow sensor 1 has a channel forming member 4 for forming a channel 3 for a fluid 2, a substrate 5 having a peripheral portion bonded to a front opening 4a of the channel forming member 4, and a plate 6 fixed (contact-bonded) to the front surface of the substrate 5 by urging it with bolts or the like through an electrical insulating film 13. In this flow sensor 1, the central portion of the substrate 5 forms a diaphragm portion 5A, and a heat-generating body and two resistors (temperature sensors) for constituting a flow rate detection sensor, and their circuit pattern 7 are formed by the known thin film forming technique.
In the flow sensor 1, the substrate 5 is formed thin, and the rear surface of the substrate 5 is in contact with the fluid 2 to form part of the channel 3 together with the channel forming member 4. As the material of the channel forming member 4 and substrate 5, a material having low thermal conductivity, high heat resistance, and high corrosion resistance, e.g., SUS304- or SUS316-based stainless steel is used.
The plate 6 has a through hole 8 having substantially the same size as that of the diaphragm portion 5A at its center. An electrode 9 is built into the through hole 8. As the electrode 9, one obtained by sealing a plurality of terminal pins 11 in a metal frame 10 with hermetic glass 12 is used. One end of each terminal pin 11 is connected to the circuit pattern 7 by brazing or soldering.
In the conventional flow sensor 1 described above, the plate 6 is merely contact-bonded to the front surface of the thin substrate 5 by fastening with the bolts. Accordingly, the mechanical and thermal contact between the substrate 5 and plate 6 is unreliable and unstable, making the temperature distribution of the diaphragm portion 5A unstable. Upon a pressure change of the fluid 2, when the diaphragm portion 5A of the substrate 5 elastically deforms in the planar direction, the contact state of the substrate 5 and plate 6 changes, and the temperature distribution of the diaphragm portion 5A changes. Then, the flow velocity or flow rate characteristics or the zero point of the sensor shifts, and the precision, reproducibility, reliability, and durability lack.
Particularly, when the interior of the channel is at a negative pressure, the substrate 5 and plate 6 undesirably separate from each other, and the flow velocity or flow rate characteristics of the sensor change largely.
Also, the number of components increases, e.g., the plate 6 and a contact-bonding mechanism for the substrate 5 and plate 6, leading to a large, complicated shape.